Hydrology and Vegetation Remote Sensing - GNSS Remote Sensing: Theory, Methods and Applications - page 244

Fig. 10.2 Specular scattering
coefficients vs. specular
angles ( upper ) and trunk
diameters ( bottom )

a Crop −−Wheat

stem dia=0.2 cm;RV pol
stem dia=0.2 cm;RL pol
stem dia=0.6 cm;RV pol
stem dia=0.6 cm;RL pol

−5

−10

−15

−20

−25

−30

−35

0 0 0 0 0 0 0 0 0

Specular Angle

θ i φ s =0¡ã

b

Forest −−Aspen−−Trunk only

15

φ s =0¡ã
φ s =60¡ã

φ s =120¡ã
φ s =180¡ã

10

5

0

−5

−10

0 0 0 0 0 0 0 0 0

Trunk Diameter(cm), θ i = θ s =30¡ã

From Fig. 10.2 , we can see that when the stem diameters are fixed, scattering
coefficients sigma is different for RV and RL pol, but when the polarization RV or
RL is fixed, different diameters have different ı ı .InFig. 10.2 , scattering azimuth
angles are considered at RV pol, as the trunk diameter increases, the specular
scattering coefficients change in different forms. Therefore, GNSS-R vegetation
biomass response is a complex process, which is influenced by observation geome-
try (azimuth and zenith angles), polarization and vegetation structure, among others.

10.3.5 GNSS-R Polarization

When GNSS signals reflected from the Earth surface, its waveform was distorted
and polarization flipped. To reduce polarization loss and receive reflected signals
efficiently, different antennas were used for different receivers: such as the modified
DMR receiver used in SMEX 02, it used the LHCP to receive reflected signals. As
mentioned above, the one used in ground-based instrument SMIGOL reflectometry
was vertical polarization. Also, linear and circular polarizations were all used in
BAO tower experiment.